1. Field of the Invention
The present invention relates to a radar apparatus of an electronic scanning method using an array antenna as a receiving antenna and, more particularly, to a radar apparatus of the electronic scanning method suited for mounting on a vehicle.
2. Related Background Art
In the array antenna, supposing the distance between the antenna elements is d and the wavelength of transmit and receive electromagnetic waves is .lambda., when d/.lambda.&gt;1, grating lobes appear in the range of the visible zone (-90.degree.&lt;.theta.&lt;+90.degree.) because of the periodicity of Fourier series. Here, .theta. is the directional angle. If the grating lobes are left in the visible zone as they are, a target might be detected erroneously. It is thus desirable to eliminate the grating lobes from the visible zone or to adequately suppress the relative power of the grating lobes with respect to the main beam.
It is possible to shift the grating lobes out of the visible zone if the distance d between the antenna elements is made sufficiently smaller than the wavelength .lambda.. The wavelength .lambda. and the distance d between the antenna elements are, however, under restrictions of other various conditions and it is, therefore, hard to change them without difficulties.
On the other hand, Japanese Laid-open Patent Application No. 6-138205 discloses the technology for providing a separate auxiliary antenna in order to suppress the receiving level of the sidelobes and it is also possible to suppress the relative power of the grating lobes by making use of this technology.
The addition of the auxiliary antenna, however, results in increasing the number of components, the size of the apparatus, and so on. This is a significant issue, particularly, in the on-vehicle radar apparatus which is desirably compact and lightweight.